The effects of local hydrodynamics on mass transfer in disordered porous media

Interfacial mass transfer in disordered media was studied experimentally and numerically. The dissolution of solid benzoic acid spheres in packed columns showed the existence of spatial variations in mass transfer coefficients in monodisperse and polydisperse packings at the same overall Peclet number. The concept of a local Peclet number (single-particle average) was introduced to quantify the effect of local hydrodynamics on local mass transfer. Correlations between Sherwood number and Peclet number having the form Shi=A·(xiPe)m were used to quantify data from various sites in each packing. These experiments also showed that the exponent m varies significantly from site to site. Stochastic simulations of interfacial dissolution in two-dimensional porous media were conducted, and mass-transfer-coefficient distributions similar to experimental results were obtained. The local velocity profiles available in the numerical simulations allowed a more detailed analysis to be made of local hydrodynamics and their effects on mass transfer. These result showed that mass transfer is affected by both large- and small-scale structure in the material. The large-scale structure affects the magnitude of local velocities and the small-scale structure (e.g., gap spacing between neighboring spheres) affects the shape of local streamlines. Particles exposed to large velocity gradients at their surface and/or particles for which streamline closely hugged their surfaces were observed to have higher rates of mass transfer. The correlation accuracy can be improved when two parameters (local Peclet number x·Pe and exponent z·m) were used in correlation such as Sh=A·(xiPe)mzi. Further analysis showed that there is a strong correlation between xi and zi and only one variable is needed to correct both the local Peclet number and the exponent term. A correlation was presented as Sh=A(xiPe)0.8965mxi-0.158 for the specific data generated in this work. In the future, the use of distributed mass-transfer correlations similar to those presented in this study may improve modeling of NAPL remediation